Electrically-propelled ships, in particular methane tankers, are known that make use of natural gas stored in liquid form as fuel.
With ships of this type, natural gas is transported in liquid form at atmospheric pressure, at a temperature of about −160° C. Although the tanks containing the cargo are thermally insulated, a fraction of the cargo evaporates on a continuous basis, typically 0.1% to 0.3% per day, due to heat passing through the insulation.
While the ship is under way, the corresponding natural gas vapor is advantageously used as fuel for providing at least part of its propulsion. Until recently, such methane tankers have been steam-propelled. The boiler delivering the steam was fitted both with burners operating on natural gas and with burners operating on heavy fuel. Depending on the energy requirements of the ship, and on the rate at which natural gas vapor is being given off by the cargo, steam production can be adjusted by using power delivered by the heavy fuel burners, in addition to the power delivered by the natural gas vapor.
That combined type of propulsion also has the advantage, whenever the energy requirements of the ship are less than the power being supplied by the boiler, of enabling excess vapor to be exhausted directly to a condenser instead of to the propulsion system of the ship, as can happen during waiting stages or when steaming slowly.
That type of propulsion nevertheless presents numerous drawbacks, and in particular;
lower efficiency than propulsion systems based on diesel oil, gas turbines, or even diesel with heavy fuel;
a machine room of large size, correspondingly reducing the volume available for cargo in a hull of given size; and
propulsion based on technology that is relatively unusual, which can lead to difficulties in maintenance and in training crew.
That type of propulsion is therefore presently in competition with techniques based on using diesel engines or gas turbines, coupled to devices that serve to burn off or to reliquefy the natural gas vapor given off by the cargo whenever the energy requirements of the ship do not enable the vapor to be absorbed. Both engines and turbines can make use either of natural gas or else of fuel oil as fuel for driving electricity generators that are used in turn, amongst other things, to power one or more propulsion motors.
When the available natural gas vapor is insufficient for providing the heat energy requirements of the ship, instead of taking additional energy from fuel oil, it can be taken from the liquefied natural gas which is pumped directly from the tanks while in the liquid state, and then vaporized.
Both natural gas vapor and liquefied natural gas are taken from the tanks at temperatures that are too cold (respectively about −130° C. and −160° C.) and at a pressure that is too low to be directly usable by the ship's propulsion system. It is therefore necessary to raise them to a temperature close to ambient and to compress them to a pressure of a few bars before using them.
FIG. 5 is highly diagrammatic and shows a prior art electrical propulsion system for a methane tanker.
The system comprises a first circuit 119 for taking the natural gas vapor 101b that is given off by the tanks 103. The natural gas vapor 101b is directed to one or more compressors 117 and then to a heating heat exchanger 115a for bringing the vapor to a pressure and a temperature that are compatible with the engines 107 of the ship's propulsion system.
In order to provide additional energy that might be needed to meet the requirements of the ship, the system includes a second circuit 121 for taking liquefied natural gas 101a, the second circuit comprising one or more pumps 123 immersed in one or more liquefied natural gas tanks 101a, and another heating heat exchanger 115b for raising the liquefied gas 105a to a temperature and pressure that are compatible with the engines 107.
The heat exchangers 115a and 115b are fed with heat from a heat source 201 (steam or electricity), thus enabling the natural gas vapor 101b and the liquefied natural gas 101a to be heated.
The ship's propulsion system comprises one or more electric motors 109b fed via power electronics 109c and driving one or more propellers 147 either directly, or via a coupler, or via a speed-reducing gearbox 145.
In addition, the motors 109b, alternators 109a, and power electronics 109c are all cooled by cooling systems 203b, 203a, and 203c using air or water.
Nevertheless, the energy efficiency of that electrical propulsion system is not very high.